Skip to main content

Natural hazards glossary

Our natural hazards glossary explains the technical terms and characteristics of a range of natural hazards in the Waikato region and elsewhere in the world.


Check out our risk mitigation plans, which minimise the effect of natural hazards on the Waikato economy and community.

Find out more about Natural Hazards and Emergency Management in the Waikato region.


Loosely compacted gravel, sand, silt, or clay deposited by streams.


Fine grained brown or greyish volcanic rock.


Igneous and metamorphic rocks that underlie the main sedimentary rock layers of a region and extend downward to the base of the earth crust.


A large volcanic crater, that usually covers an area greater than the vents within it.

Catchment wetness indicator

A method for measuring how wet the ground is which is then used as a basis to indicate the absorptive qualities of the catchment (i.e. how much more rain can the soils absorb before runoff occurs). This is obtained by continuously measuring rainfall within the major tributaries and calculating a value (or an index) which is known as the Antecedent Precipitation Index (API).


Rock made up of small stones held together.


The point at which an earthquake reaches the earth’s surface (the central point of an earthquake’s source).


A fumarole (or fumerole) is a hole from which superheated gas and steam discharges under pressure. It is a feature in geothermal areas.

HectoPascals (hPa)

Units used to measure air pressure. Air pressure is the weight per unit area exerted by the air above. It can be measured in hectoPascals using a barometer.


A term used to describe rocks that have formed or faults which have occurred 10,000 years or less before the present day. Faults of this age are commonly considered active, based on the observation of historical activity on faults of this age in other places.

Igneous rocks

Igneous rocks make up most of the earth's surface, and can form in two ways:

  • from (mainly) silicate magma forced from a volcano or a volcanic vent (‘extrusive’)
  • within the earth’s crust as a molten body (‘intrusive’), which becomes visible on the surface when the overlying sediment and soil has been eroded away.

Both forms of magma cool and form igneous rocks. One example is of igneous rock found in the Waikato Region is andesite.


A type of unstable clay often formed from volcanically derived sediments.


Mudflow comprised of mainly volcanic debris.


A soft, brown coal showing traces of plant structure. This coal is an intermediate stage between bituminous coal and peat.


Process by which water-saturated sediment temporarily loses strength and acts as a fluid. This effect can be caused by earthquake shaking.


The outer solid part of the Earth that includes the crust and uppermost mantle. The lithosphere is about 100 km thick, although its thickness is age dependent. The lithosphere below the crust is brittle enough at some locations to produce earthquakes by faulting, such as within a subducted oceanic plate.


Fluid or semi-fluid material under the Earth’s surface from which lava and other rock is formed by cooling.

Metamorphic rock

Metamorphic rock is formed when either sedimentary or igneous rock is altered by heat, pressure and/or chemical reaction (with the exception of chemical weathering). One example is quartzite.


Either activities directed towards eliminating or reducing the probability of occurrence of a disaster-producing event, or reducing the effects of those events that are unavoidable.


The Modified Mercalli Intensity Scale (usually just abbreviated as MM) refers to the relative amount of damage that structures undergo during an earthquake. This subjective measurement tool varies from country to country, as it relies on building construction quality which cannot be easily assessed. The scale used in New Zealand ranges from MMI to MMXII.

Plate tectonics

A theory supported by a wide range of evidence that the Earth's crust and upper mantle are composed of several large, thin, relatively rigid plates that move relative to one another. Slips, which occur on faults defining plate boundaries, commonly result in earthquakes. Several styles of faults form the boundaries of the plates, including:

  • thrust faults - where plate material has moved beneath or been consumed within the mantle
  • oceanic spreading ridges - where new earth crust material has formed
  • transform faults - these make room for material from horizontal slip (strike slip) between adjoining plates.

Pyroclastic flows

A dense mass of very hot ash, lava fragments, and gases ejected from a volcano and often flowing at great speed.


The geologic time period comprising about the last 1.65 million years.

Richter Scale

A logarithmic scale of 0 to 10 representing the strength of an earthquake. Severe earthquakes have magnitudes greater than 7.

Rhyolite domes

Granite-like land forms created through a build up of fine grained volcanic rock.

Sedimentary rock

Sedimentary rock is formed when pre-existing material builds up (such as sand or landslide debris) and hardens (‘consolidates’). One common example of a sedimentary rock found in the Waikato Region is greywacke, which is consolidated sandstone.


Geothermal area where water heated from hot rocks beneath the earth’s surface rises through cracks and other small openings in the ground.


Oscillation of the surface of an enclosed body of water owing to earthquake shaking.


A mineral crust or deposit formed from the minerals (mainly silica) in geothermal water, especially from geysers.


A plate tectonics term for the process through which an oceanic lithosphere collides with and descends beneath the earth’s continental lithosphere.


Refers to the processes in which rock is broken down, affecting structures over regional sections of the lithosphere. Also used as a term to identify where there has been a structural trigger for an earthquake or tsunami.

Refers to rock-deforming processes and resulting structures that occur over regional sections of the lithosphere. Also relates a particular phenomenon to a structural concept, such as the trigger for an earthquake or a tsunami.


Maori term for a sinkhole or entrance to a cave or hole in the ground. Tomo are formed when groundwater dissolves underlying limestone rock. Commonly found in ‘karst’ landscapes (limestone) such as that in the Waitomo District in the Waikato Region. ‘Wai’ = water, ‘tomo’ = entrance: thus ‘place where water enters the ground’ (water entrance).


The time taken for a tsunami wave front to travel from the triggering source to the coastline.


A Japanese word made up from two characters. The character ‘tsu’ means harbour and the character ‘nami’ means wave. A tsunami occurs as a series of travelling ocean waves of extremely long wavelength and is triggered by large disturbances such as earthquakes, volcanic eruptions or deep sea landslides.

Wave amplitude

The height of a wave from the still water level to the top of the wave crest.

Wave crest

The highest part of the wave above the still water level.


The horizontal length of a wave in the direction of motion. An ocean wave is made up of a wave crest and a wave trough The wavelength is the distance between successive points of equal amplitude and phase on a wave (for example, crest to crest, or trough to trough).

Wave period

The time it takes for two successive wave crests or troughs to pass a fixed point in the ocean.

Wave trough

The lowest part of the wave.

Weather Bomb

A low pressure system which rapidly deepens causing barometric pressure to drop by at least 25 hPa in a 24 hour period. These systems occur when the normal processes for developing depressions become intense. These processes include the effects of strong contrasts between air masses (cold or warm fronts) and the release of energy when the uplifting of moist air forms clouds.

The deep and intense nature of weather bombs leads to large areas of gale force winds and intense rainfall. This type of weather system can rapidly form over or near the North Island about once every two years, but few go develop into full-blown 'weather bombs'.